The present invention relates to a system to detect and correct system-introduced distortions to chirp and stretch waveforms used in radar systems.
Chirp or stretch waveforms, which employ linear ramp frequency modulation formats, are frequently used to provide pulse compression for radar range resolution enhancement. Good performance from these waveforms is obtained only if phase and amplitude distortions introduced in the waveform signal path are controlled. Such distortions arise from the exciter output circuitry, the radar transmitter, and the receiver circuitry.
A known technique to generate linear, wideband chirp and stretch waveforms includes the exciter output circuitry and transmitter in the waveform generation and linearization process. With the technique, the basic ramp frequency modulation is applied to an open loop voltage controlled oscillator (VCO). Using a delay line frequency discriminator as the modulation error measuring device, a feedback loop is closed around the discriminator, VCO, exciter output circuitry, and transmitter. The instantaneous slope error of the transmitter output waveform is measured by the discriminator, converted into a voltage, and summed with the open VCO modulation to effect the linearization. That technique requires a complex feedback control loop that typically is divided between separate hardware units and is severely limited in the range of waveform slopes that it can accommodate with a given delay used in the discriminator.
A second previous technique, described in U.S. Pat. No. 4,359,779, accomplishes the correction by enclosing the transmitter and exciter output circuitry in phase sensitive and amplitude sensitive feedback control loops. A phase sensitive detector generates a phase error signal by comparison of a sample of the transmitter output waveform with a precisely delayed sample of the waveform generator output. That error signal is applied to a phase modulator for correction. Precise delay equalization between the waveform generator and transmitter sample signals is required to minimize the phase range required for the phase modulator. The amplitude control loop compares the detected amplitude of the transmit signal with a reference voltage and applies the resulting error voltage to the transmitter driver for correction. The feedback action maintains a constant (or other desired profile) pulse amplitude. This approach require two feedback control loops, precise delay equalization, a high performance phase modulator, and considerable other electronics to insure that the errors at the beginning of each pulse are zeroed to prevent large waveform errors due to starting transients and the resulting loop setting.
While those techniques are effective, substantial additional hardware is required and the feedback loop are generally complex. The feedback control loop(s) are also typically closed around circuit functions that reside in different units, making test and fault isolation difficult.
It is therefore an object of this invention to provide a system for transmitter phase and amplitude correction for linear FM systems which requires little additional hardware, and does not require complex or high precision circuitry.
A further object is to provide a correction system which corrects for FM system receiver distortions ahead of the first downconversion mixer.
Yet another object is to provide a phase and amplitude distortion correction system which provides correction data in a digital format, and thereby permits processing to easily shift the data set to obtain a zero first correction to avoid the introduction of waveform starting transients or to digitally filter the data to effect smoothing of the applied correction.